Compressor control



Feb. 3, 1970 G J. AKERHIELM ETAL 3,493,157

COMPRESSOR CONTROL Filed Aug. 16, 1967 INVENTORS. GEORGE J. AKERHIIELM.BY JAMES F. WAKE.

ATTORNEY.

I United States Patent "ice US. Cl. 230-4 3 Claims ABSTRACT OF THEDISCLOSURE A control system for regulating centrifugal compressorcapacity by varying compressor speed to effect a first range ofreduction in capacity and thereafter maintaining compressor speedconstant and varying compressor guide vane adjustment to effect anadditional capacity reduction.

BACKGROUND OF THE INVENTION In many centrifugal gas compressorinstallations, such as refrigeration systems where the compressor isemployed to pump refrigerant in the gaseous state from low side pressureto high side pressure, a variable load on the compressor is encountered.In order to maintain compressor efliciency and prevent compressor surge,it has been the practice to vary compressor capacity. This isaccomplished by varying the position of inlet guide vanes and/ordiffuser vanes or by varying compressor speed. At high compressoroutputs, the most efficient way of varying capacity is to varycompressor speed. However, if compressor speed is dropped below acertain point, compressor surge may occur. To avoid this problem movableguide vanes are utilized to regulate compressor capacity, resulting in acompressor having a wide stable operating range, at the expense ofcompressor efficiency.

Another problem encountered in compressor operation is the danger ofoverloading the driver under extreme operating conditions. For example,in a refrigeration system employing a centrifugal compressor the load onthe compressor at start-up can exceed the design operational load to anextent which could damage the driver.

SUMMARY OF THE INVENTION This invention relates to a compressor controlsystem to vary compressor output according to the load thereon byutilizing a load controller to provide a signal proportional to the loadwhich is effective to alter the set point of a constant speed governorover a range of high loads and which is effective at a preselectedmedium loading to maintain compressor speed constant While activating acompressor guide vane controller so as to vary compressor capacity bythe variation of inlet guide vanes over a range of low compressor loads.

BRIEF DESCRIPTION OF THE DRAWING The figure is a schematic diagram of acontrol system for a turbine driven centrifugal refrigeration system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thereis illustrated a steam turbine 3 driving a centrifugal refrigerantcompressor 5, which supplies refrigerant to condenser 7, expansion valve9 and water chiller 11. From water chiller 1-1, the refrigerant isreturned to the compressor inlet which has adjustable guide vanes 13therein. The turbine is supplied with motive fluid through line 15 whichis regulated by governor valve 17 to control turbine speed. The governorvalve 17 is comprised of a positive displacement pump 19 operablyassociated with the turbine shaft so as to 3,493,167 Patented Feb. 3,1970 provide fluid under pressure, proportional to turbine speed, to adiaphragm 21, which, through linkages 23, controls the positioning ofthe valve disc 25 to regulate the motive fluid supplied to the turbineso as to maintain a constant speed thereof. The initial setting of valve17 is obtained by the adjustment of a needle valve 27 which bleeds aportion of the pumped fluid from the governor valve circuit so as tolimit the pressure obtainable therein.

The control system is provided with a temperature controller 29 which isresponsive to the temperature of the chilled water leaving chiller 11 asevidenced by chilled water sensor 31 to vary control air pressure from asuitable source (not shown) so as to provide control air pressurebetween 3 p.s.i. and 15 psi to the control system. Under high loadconditions evidenced by a control pressure range between 15 p.s.i. and 9p.s.i., the capacity of the compressor is varied by changing the speedof the turbine. This is accomplished by a secondary bleed valve 33 whichis responsive to control pressure transmitted thereto to change thegovernor valve set point as evidenced by bleed valve 27 by bleedinggovernor valve pressure therefrom. Secondary bleed valve 33 is sized soas to be wide open at 9 psi. at which point the pressure in the governorvalve circuit is such as to provide a bottom limit to compressor speed.When the system capacity is being varied by changing turbine speed andthus the pumping rate of the compressor, it is also subjected to anoverload control comprised of an electropneumatic transducer 35responsive to turbine speed as evidenced by an electrical signalproduced by a signal generator 37 on the turbine drive shaft whichtransmits a pneumatic signal to a differential controller 39' includingupper and lower bellows elements 38 and 40. As may be noted in thedrawing lower bellows 40 communicates with the output of the transducer35. System control pressure from the temperature controller 29 isprovided to the upper bellows element 38 of the differential controller39. As long as the two signals provided to the differential controllerare equal, av constant 9 p.s.i. signal is transmitted through athree-way solenoid valve 41 to guide vane regulator 43 which issuflicient to maintain the vanes wide open. If the signal from thetransducer 35 drops below that from temperature controller 29, itindicates that the turbine is overloaded. Differential controller 39senses the difference in the signals being applied to the bellowselements and automatically throttles the signal to the guide vaneregulator 43 by transmitting a reduced pressure signal to the regulator,partially closing the vanes, unloading the compressor and permitting theturbine to return to the desired speed. During high load operation,control pressure from temperature controller 29 is also utilized to holdswitch 45 open so as to maintain solenoid operated three-way valve 41 ina position to communicate'differential controller 39 with guide vaneregulator 43.

Once the signal from temperature controller 29 drops below 9 psi, it isineffective to further vary the governor set point. As such, below thatpoint, the governor will act to maintain turbine speed constant at thelevel dictated by a 9 psi. signal. A control pressure below 9 psi. willallow switch 45 to close, activating three-way solenoid valve 41 so asto communicate control pressure directly to guide vane regulator 43. Thelow range system capacity, therefore, is varied solely by thepositioning of the guide vanes 13 under the influence of controlpressure from temperature controller 29 below the 9 p.s.i. level.

As can be seen from the above, applicants compressor control providesfor economical operation of a wide range of compressor loads andincludes a provision to prevent driver overload at extreme high loadconditions such as start-up.

While we have described a preferred embodiment of g the invention, it isto be understood the invention is not limited thereto.

We claim:

1. A control system for a gas compressor having a variable speed drivercomprising:

load sensing means operable in response to compressor loading to providea signal proportional thereto; means for varying the speed of thecompressor in response to the signal from said load sensing means; speedsensing means operable in response to the speed of the compressor toprovide a signal proportional thereto;

means associated with the inlet of the compressor to vary the capacitythereof; and

differential control means operable in response to the differencebetween the signal from said load sensing means and the signal from saidspeed sensing means indicative of an overload on the driver to actuatesaid compressor inlet means to vary compressor capacity to preventoverload thereof.

2. A control system for a gas compressor according to claim 1 whereinthe variable speed driver comprises a steam turbine,

said load sensing means including a pneumatic control operable inresponse to compressor loading to provide a variable pressure airsignal,

said means for varying compressor speed including a steam governorvalve, operable in response to air pressure from said pneumatic controlto vary the quantity of steam supplied to the turbine,

said speed sensing means including a signal generator,

operably associated with the compressor to provide an electric currentproportional to compressor speed, said speed sensing means furtherincluding an electro-pneumatic transducer for providing a variablepressure air signal in response to the variations in electric currentfrom said signal generator.

3. A control system for a gas compressor according to claim 1 whereinsaid means for varying the speed of the compressor is nonresponsive to arange of signals from said load sensing means indicative of lowcompressor loading, the control system further including means,responsive to a range of signals from said load sensing means indicativeof low compressor loading to bypass said speed sensing means and saiddifferential control means to eifect control of said compressor inletmeans directly by said load sensing means.

References Cited UNITED STATES PATENTS 1,381,513 6/1921 Smith l03122,385,664 9/1945 Warner 230-1 14 2,397,443 3/ 1946 Statham et al 103162,401,910 6/1946 Condit 2307 2,632,307 3/1953 Massey et al. 230112,941,120 6/1960 Harmon 10335 3,226,011 12/1965 Gustafson 2302 3,248,0434/1966 Taplin 230-115 WILLIAM L. FREEH, Primary Examiner US. Cl. X.R.230--11

